1. Field of the Invention
This invention relates to improved thermoplastic composites and to methods of preparing same. The improved thermoplastic composition which comprises a thermoplastic polymer having fibrous reinforcement dispersed therein. The present invention also relates to producing prepregs used in the formation of laminates and pultrusions and filament wound articles (all forms of composites) having high performance fibers and polymeric matrices.
2. Prior Art
Prepregs are a well known intermediate form in the preperation of composites having high performance fibers and polymeric matrices. In the most common form, a prepreg contains a series of parallel fibers (such as boron or carbon or glass fibers) held together with a thermosetting polymer such as an epoxy resin. One method for forming prepregs is to size (impregnate the interstices of) a multifilament bundle of high performance fibers with a dilute solution of the resin, and then coat the sized fibers with a melt or concentrated solution of the resin. Thereafter, the coated fibers are layered up in parallel fashion; and the resultant sheet is dried in order to produce a sheet which holds together reasonably well in the transverse direction. Subsequently, a series of such sheets are stacked with the fiber direction varied in a regular manner, and the assemblage is cured either in a closed mold or in an autoclave under increased temperature and superatmospheric pressure.
Pultruded articles are composites prepared by aligning high performance fibers coated with a thermosetting resin in a substantially parallel linear array and curing the assemblage. Examples include structural parts such as I-beams. Filament wound articles are formed by winding similar coated fibers in multiple layers on a substrate (e.g., a mandrell or a part) and curing the assemblage. Pultrusion and filament winding processes may differ from prepreg processes in that in the pultrusion and filament winding processes, a prepreg is not isolated and arranged into a desired shape before curing. When fibers for pultrusions or filament winding are coated from solution, the drying step may overlap both with the aligning step and the curing step.
It has been proposed that fiber filled composites can be formed by passing roving that is spread out to expose its multiple filaments through a fluidized bed containing resin microparticles of the same diameter as an individual filament (8 to 20 microns). The roving, which has been pretreated with sizing to temporarily retain the resin particles in place is then consolidated and sheathed with an extrusion coating of the same resin. This process is claimed to provide a more uniform resin content. Several disadvantages flow from this process. For example, air pockets are trapped in the rovings which are difficult to remove. The trapped air results in voids in the filled composition resulting in a reduction of strength. Moreover, the use of the sizing may be detrimental to forming a strong interface bond between the resin and the fiber, and precludes any tailoring of the sizing to the type of fiber and resin.